1. Field of the Invention
This invention relates to a liquid crystal display system, and more particularly to a double-layer type super-twisted nematic liquid crystal display system.
2. Discussion of the Related Art
A conventional display means employed in an automobile's head-up display system is of a seven-segment type using fluorescent display tubes, because the display must be high in luminance. The fluorescent display tubes of the segments are selectively turned on to display, for instance, a vehicle speed and the number of revolutions per minute.
In the display means, the fluorescent display tubes emit light high in luminance. Hence, the display means is advantageously employed, for instance, for an automobile's head-up display system which makes display by reflecting light with a combiner. However, since the display means is of a seven-segment type, it is limited in the form of display; that is, what can be displayed by the display means is mainly numerical data. More specifically, it is unable for the display means to display a variety of data including characters and graphic forms. That is, the display means is rather inadequate as means for displaying a variety of pieces of information.
In order to eliminate this difficulty, a dot-matrix type transmission liquid crystal display device is employed as the display means in the head-up display system. In this case, in order to have a clear display, it is essential to use a liquid crystal display device which is able to provide a display sufficiently high in contrast.
There are available a variety of liquid crystal display devices, such as, a twist nematic type liquid crystal display device, a ferroelectric liquid crystal display device, and a phase transition type liquid crystal display device. Among those display devices, for instance, a super-twisted nematic liquid crystal device is considered to be relatively high in contrast. However, maximizing the display contrast of the super-twisted nematic liquid crystal device gives rise to various problems as follows:
FIG. 1 shows a conventional super-twisted nematic liquid crystal display system. The display system includes: a dot-matrix type super-twisted nematic liquid crystal display device (hereinafter referred to merely as "a liquid crystal display device", when applicable) 1; and a compensating liquid crystal device 2. The compensating liquid crystal device 2 optically eliminates a coloring phenomenon in the liquid crystal display device 1 which is caused by the phase difference in the direction of advance of light therein; that is, the device 2 makes the display monochromatic, thereby to increase the contrast. The twist angle of the compensating liquid crystal device 2 is opposite in direction to the twist angle of the liquid crystal display device 1. The display system further includes: a drive voltage control circuit 3 for adjusting a drive voltage applied to the liquid crystal display device 1; a temperature sensor 4 for detecting the ambient temperature of the liquid crystal display device 1; a display control circuit 5 for allowing the liquid crystal display device 1 to display characters and/or graphic forms; and a light source 6 which irradiates the liquid crystal display device 1 in a transmission mode.
In the display system thus organized, in response to signals from the display control circuit 5, the liquid crystal display device 1 displays characters and/or graphic forms. The display on the display device 1 is irradiated by the output light of the light source 6 in a transmission mode. When the display thus irradiated passes through the compensating liquid crystal device 2, the light twisted by the liquid crystal display device 1 is twisted back, which eliminates the elliptic light polarization attributing to the phase difference, and the variations in direction of the light polarization due to the wavelength of light. As a result, the display is made monochromatic; that is, the coloring phenomenon is eliminated, so that the display is improved in contrast.
In the conventional liquid crystal display system, the display (or image) on the liquid crystal display device 1 has a density characteristic as shown in FIG. 2.
That is, in general, in order to make a display high in contrast, the following expressions must be satisfied: EQU V.sub.on .gtoreq.V.sub.th +.DELTA.V EQU V.sub.off .gtoreq.V.sub.th
where V.sub.th is a threshold voltage with which the liquid crystal starts to provide an electro-optic effect, V.sub.on is an effective voltage of a selected point (or a light-on picture element), and V.sub.off is an effective voltage of a not-selected point (or a light-off picture element).
This will be described with reference to FIG. 2 in more detail.
As the applied voltage is gradually increased from zero, the selected point is increased in image density. When the applied voltage is further increased, the not-selected point is also increased in image density. If it is assumed that the selected point has an image density of 10% with a voltage V.sub.th1, and an image density of 50% with a voltage V.sub.50, and the not-selected point has an image density of 10% with a voltage V.sub.th2, then the voltages with which an image high in contrast can be displayed are ranged from V.sub.50 to V.sub.th2. This is generally called an "operating voltage range".
Thus, in order to improve the display contrast, the applied voltage should be set to a value in the above-described operating voltage range. On the other hand, in the case of a liquid crystal display system which is generally used, the above-described image density characteristic depends on the ambient temperature of the liquid crystal display device; that is, it is on the low voltage side when the ambient temperature increases, and it is on the high voltage side when the ambient temperature decreases. In order to overcome this difficulty, a temperature sensor such as a thermistor is employed to perform a temperature compensation according to the temperature characteristic of the drive voltage of the liquid crystal display device. However, since the temperature characteristic of the drive voltage is non-linear, the adjustment of the temperature characteristic by using the temperature sensor such as a thermistor is limited. On the other hand, the threshold voltage of the liquid crystal display device drifts, which makes it difficult to provide a suitable drive voltage, and accordingly to display a high contrast image.